1. Fields of the Invention
The present invention relates generally to an apparatus and method of measuring hemispherical light scattered or emitted from a source, and, more particularly, to a portable scatterometer which uses a double tapered fiber optic bundle with a concave spherical face, a CID camera, and a frame grabber to hemispherically collect scattered light reflected from a laser illuminated sample and a unique algorithm to rapidly reconstruct the scatter profile on a computer screen.
2. Discussion of Background and Prior Art
a. Scatterometers
Scatter from optical components reduces signal power, limits resolution, produces noise and has appeared as an unexpected problem in more than one optical design. Stover, "Optical Scatter: Careful Measurement Of Optical Scatter Provides A Keen Diagnostic", Lasers & Optronics, August 1988.
Optical designers and manufacturers require a precise and fast hemispherical light scatter measurement tool because many optical surfaces interact with light in unpredictable ways.
A scatterometer is a widely used and extremely valuable tool for optical designers, measuring scattered light from test objects in order to determine the quality and characteristics of surfaces down to the angstrom level.
In 1987 Breault Research Organization, Inc. ("BRO") introduced a multi-wavelength, surface-scanning, fully automated scatterometer ("FASCAT 360") system for use in research and development markets. The system could accommodate up to seven lasers and obtained full hemispherical measurements of the light reflected from or transmitted through a sample, but, only by rotating a photosensitive detector 360 degrees about the sample holder while allowing for three-axis (X,Y and Z) rotation and translation of the sample itself (all automated). The instrument is a Bi-Directional Reflectometer ("BDR") capable of measuring BiDirectional Reflectance Distribution Functions ("BRDF") and BiDirectional Transmittance Functions ("BTDF") both in-plane and out-of-plane. The system printed 2-D and 3-D plots of the data in real time and provided unparalleled versatility and dependability.
Until recently, this now conventional technology has limited the range of applications of scatterometers because of its large size (a 4'.times.8' steel top table completely encased in a Center For Radiological Devices and Health Class 1 housing, called the "truck", with sample access through safety interlock doors and a 386SX computer/laser printer system), its high cost ($400,000-$600,000) and its long acquisition time (10 minutes to 3 hours to measure, calculate and print a complete analysis of a sample).
To overcome this size and cost disadvantage, in 1988 Toomay, Mathis & Associates, Inc. ("TMA") introduced a single laser, table mounted, Complete Angle Scatter Instrument ("CASI".TM.) Class 3 scatterometer, but at great sacrifice to versatility and dependability. While this instrument provided three axis rotation and translation (one of which was automated) of the sample and 360 degree "sweep" (a different technique than 360 degree rotation about the sample used by FASCAT) of the detector, it could only provide 2-D plots of BSDF and still was relatively expensive ($97,000-$166,000).
Even more recently, still lower cost, hand-held, battery-powered, microprocessor-controlled scatterometers have been introduced using hand-held or bench-mounted measurement heads with up to 8 individual, non-movable detectors spaced about the sample which restricted versatility even more and which further sacrificed reliability and volume of data. While performing some useful function, these smaller units are restricted in that they assume the surface to be measured is homogeneous and that there is no interaction of light which prevents homogeneity. Thus, they are restricted in the number of measurements they can make.
The above attempts to design smaller instruments led to instruments that had much less capability than full scale versions, and, as a result, important information was lost. Applicant has found a solution to this problem which minimizes the sacrifices made in capability to achieve the small size and yet can measure scattered light on un-nice, non-behaved, non-homogeneous surfaces as did full scale versions, and, not only, without sacrificing versatility and capability, but rather, increasing it. Moreover, none of the prior scatterometers are capable of spherically simultaneously measuring the angular gradation of the light scattered from a source because none of the light collection systems in any of the prior scatterometers were capable of spherically, simultaneously collecting the reflected light scattered from a spot on a surface of the sample. Applicant's unique collection system has solved this problem.
b. Fiber Optic Bundles
Fiber optic bundles have been known for many years. See, U.S. Pat. Nos. 2,354,591 and 3,033,071 and Siegmund, "Fiber Optic Tapers in Electronic Imaging", Schott Fiber Optics.
A modern fiber optic bundle comprises millions of individual fibers of glass which are first made by pouring pure raw glass of high index of refraction into a tube of lower index of refraction cladding glass, and which are then precisely aligned and fused together to form a solid fiber glass bundle ("boule"). Each fiber sees and carries one small portion of the image by the well known process of internally reflecting light rays emanating from the image. Through this process high resolution images may be efficiently transferred from one surface to another.
During the manufacturing process it is also well known to twist, bend or taper the boule depending on the end function desired. The taper, for example, is made by heating the center and pulling the ends to produce an hour-glass shaped boule with the fibers essentially parallel at the larger diameter ends and smaller diameter center of the boule. During this process the outermost fibers are stretched more and are longer than the innermost fibers. The boule is then cut in half at the small diameter center to provide two identical tapered halves, each of which becomes a fiber optic magnifier/minifier.
Faceplates serve as windows and transmit the image straight through without changing the size or orientation. Twisted bundles function as image inverters. Tapers serve as magnifiers or minifiers. The two end faces of the bundles are preferably parallel planes and may be flat or curved to a desired radius. It is well known to couple the small end of the taper to a self scanned array, such as, a charge coupled device ("CCD") to convert the light level in a group of fibers or "pixels" to a corresponding electrical signal which can be digitized and reconstructed graphically as an intensified image on a computer screen, for example in spectroscope, astronomical and medical applications.
Fiber optic bundles have found wide use in such fields as x-ray image intensifiers and night vision goggles, for example.
As disclosed in U.S. Pat. No. 3,033,071, it is known to further heat segments of the boule and pull the ends of the fibers to form a double tapered, onion shaped boule which is then cut at a point in the tapered portion to form concave surfaces in one or both ends for use as an image or field flattener. In this early device, however, the image is not of a point which is the focal point of the bundle, normal to all fibers and which is a light or a scattered light source that radiates light such that the radiated light strikes the bundle at 0.degree. incidence along the entire surface of the bundle.
As disclosed in U.S. Pat. No. 4,991,971, it is known to have a bundle of equal length optical fibers each end of each of which is arranged in a circular array equidistant from the object being tested and the other ends of which are in a linear array whereby each fiber simultaneously receives a different angular component of the scattered light at the one end and transmits it to the other end such that the transmitted components exit the linear array end simultaneously and are detected and converted to electrical signals by a computer. The constraint of equal length fibers prevents use of a tapered fiber optic bundle in this system. Moreover, the device is limited to reading only that portion of globally scattered light that appears in the single plane in which the circularly spaced fibers are located. Thus, the collection and computation of a scatter profile for a spherical segment or a full hemisphere requires rotating the sample as in other prior schemes with resultant lengthy, slow construction of the scatter profile.
These deficiencies are overcome in the present invention through the use of a tapered fiber optic bundle, each fiber element of which receives light normal to its aperture from the light source which is at the focal point of the bundle, and which has never before been used in measuring light in a scatterometer. Moreover, a double tapered fiber optic bundle of applicant's unique design provides the remarkable advantages of instantaneous, spherically segmented or hemispherical collection of light from a scatter source and has not been heretofore known.
c. Frame Grabber Algorithms
A frame grabber, or image memory, has been used in the past as part of an image sensor processor. See, U.S. Pat. Nos. 5,040,116, 4,954,962 and 4,843,565. Typically, the frame grabber is contained within a frame grabber pc-board, such as a type made by Coreco or Image Technologies, and is coupled to a data processing device, such as, an 80486 Intel microprocessor driven computer, which, accesses the image memory and, according to a predetermined algorithm, reconstructs the image on a cathode ray tube or other luminescent screen. A standard frame grabber is capable of resolving 256 shades of grey. Where the ambient light has to be eliminated or otherwise adversely influences the reconstruction of the subject image, it has also been known to use a technique of subtracting out the ambient light value from the data. See U.S. Pat. No. 4,991,971 (4:56-64).
Prior frame grabber algorithms have been slow and inefficient and have required the use of expensive CCD cameras. The advantage of applicant's unique algorithm is that its high level of efficiency enables the use of a low cost charge injection device ("CID") camera which eliminates the significant "blooming" problem experienced with CCD cameras when pixels become saturated and which prevents good scatter profile reconstruction.